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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"zero() returns error if there are no zeros and slycot is installed","articleBody":"PR #205 added support for MIMO systems in the calculation of zeros for state space systems (using `slycot`), but there is a bug if there are no zeros in the input/output response.\r\n\r\nTo generate the bug, use\r\n```\r\nimport control\r\ns = control.ss( control.tf([1], [1, 2, 1]) )\r\ns.zero()\r\n```\r\nWithout `slycot`, we get\r\n```\r\narray([], dtype=float64)\r\n```\r\n\r\nWith `slycot` (0.3.3) this generates the following error trace:\r\n```\r\n~/src/python-control/murrayrm/control/statesp.py in zero(self)\r\n    526                          self.A, self.B, self.C, self.D)\r\n    527             nu = out[0]\r\n--\u003e 528             return sp.linalg.eigvals(out[8][0:nu,0:nu], out[9][0:nu,0:nu])\r\n    529         except ImportError:  # Slycot unavailable. Fall back to scipy.\r\n    530             if self.C.shape[0] != self.D.shape[1]:\r\n\r\n~/anaconda/envs/python3.6-slycot0.3.3/lib/python3.6/site-packages/scipy/linalg/decomp.py in eigvals(a, b, overwrite_a, check_finite, homogeneous_eigvals)\r\n    767     return eig(a, b=b, left=0, right=0, overwrite_a=overwrite_a,\r\n    768                check_finite=check_finite,\r\n--\u003e 769                homogeneous_eigvals=homogeneous_eigvals)\r\n    770 \r\n    771 \r\n\r\n~/anaconda/envs/python3.6-slycot0.3.3/lib/python3.6/site-packages/scipy/linalg/decomp.py in eig(a, b, left, right, overwrite_a, overwrite_b, check_finite, homogeneous_eigvals)\r\n    224             raise ValueError('a and b must have the same shape')\r\n    225         return _geneig(a1, b1, left, right, overwrite_a, overwrite_b,\r\n--\u003e 226                        homogeneous_eigvals)\r\n    227 \r\n    228     geev, geev_lwork = get_lapack_funcs(('geev', 'geev_lwork'), (a1,))\r\n\r\n~/anaconda/envs/python3.6-slycot0.3.3/lib/python3.6/site-packages/scipy/linalg/decomp.py in _geneig(a1, b1, left, right, overwrite_a, overwrite_b, homogeneous_eigvals)\r\n     78     ggev, = get_lapack_funcs(('ggev',), (a1, b1))\r\n     79     cvl, cvr = left, right\r\n---\u003e 80     res = ggev(a1, b1, lwork=-1)\r\n     81     lwork = res[-2][0].real.astype(numpy.int)\r\n     82     if ggev.typecode in 'cz':\r\n\r\nValueError: failed to create intent(cache|hide)|optional array-- must have defined dimensions but got (0,)\r\n```\r\n","author":{"url":"https://github.com/murrayrm","@type":"Person","name":"murrayrm"},"datePublished":"2018-07-07T20:16:39.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/218/python-control/issues/218"}